Summary:
HisPMQJ is an ATP-dependent histidine transport system that is a member of the ATP-Binding Cassette (ABC) Superfamily of transporters [Wu95]. Based on sequence similarity, HisJ is the periplasmic histidine-binding protein, HisQ and HisM are the integral membrane components, and HisP is the ATP-binding component of the ABC transport complex [Liu98a]. Although the transport properties of the His complex in E. coli have not yet been characterized, extensive investigations on the orthologous proteins in Salmonella typhimurium have been reported. The His complex of Salmonella typhimurium was purified and reconstituted into ATP-encapsulated proteoliposomes, and histidine transport activity was observed [Liu97d]. His-mediated transport activity is completely dependent on the presence of all four protein components and on the internal ATP concentration, with apparent Km of 8 mM for ATP [Liu97d, Bishop89]. The transport activity is also affected by pH, temperature, and salt concentration [Liu97d]. Transport is irreversible and accumulation reaches a plateau at which point transport ceases [Liu97d]. The transport complex is inhibited by ADP and by high concentrations of internal histidine [Liu97d].